Well, I beleive that the first thing you should know will be the programming of the computer, how will the computer receive data thorugh parallel or serial ports, and how to interpret the data. Then sending data to the device, e.g microcontroller, either serial or parallel. Then thats the time you will program the microcontroller. You can either start at computer or microcontroller side, just as long as you know how the process both side should do and the limitations your process you run into...